The present invention relates to a way of positioning optically an object which has a register mark on it, and in particular to an automatic electro-optical system for positioning an object such as a printing film or plate by detecting a register mark thereon using photoelectric elements.
In a conventional electro-optical positioning system, such as disclosed in U.S. Pat. No. 3,544,801, a first object having a first register mark is positioned with respect to a second object having a second register mark by detecting the differences of output signals from a photo-detector which comprises four photoelectric elements 1, such as shown in FIG. 1, each of which occupies one of four divided surfaces of the photodetector, and which are arranged in a cross-shape, divided by a cross-like element 2. This produces an equal output signal when equal areas of the elements 1 receive equal amounts of light from a light source.
In this prior art, it is necessary to have photoelectric elements which have the same performance characteristics, which is not convenient in practice.